With an electric mixer, beat the egg whites to stiff peaks. Set aside.
In a separate bowl, beat together the egg yolks, milk, vanilla and oil.
Add the dry ingredients and mix to combine.
Gently fold in the egg whites until mixed – try to do this gently so they keep some air and help make the waffles fluffy.
Scoop 1/4 cup of batter into the waffle iron. Sprinkle with blueberries and cook until the waffle iron indicates they're done (every iron is different!).
Repeat with the rest of the batter and blueberries.
Enjoy warm with maple syrup and butter/yogurt.
Notes
If you want to add more protein to these waffles, swap 1/3 cup of milk with 1/3 kefir or 1/2 cup greek yogurt.